1. Field of the Invention
The present invention relates to a developer carrier, a developing device, an image forming apparatus and a computer system.
More specifically, the present invention relates to a developing device used in an image forming apparatus, in which toner is attached onto an electrostatic latent image formed on an image carrier to make the image visible.
2. Related Art
As this type of image forming apparatus, image forming apparatuses are known, for example as disclosed in Japanese Patent Publications Nos. JP S63-58384A and JP H10-48933A, that include plural developing devices that develop a latent image formed on a photosensitive body as an example of an image carrier with toner as an example of a developer, and which are disposed with a developing unit of a rotary format where these developing devices are radially disposed around a rotary shaft. In these image forming apparatuses, when an image signal is sent from an external device such as a host computer, the developing unit is rotated around the shaft, whereby one of the plural developing devices is positioned at a development position opposing the photosensitive body. Additionally, the latent image formed on the photosensitive body is developed to form a toner image and transferred onto an intermediate medium. At this time, the plural developing devices are successively switched to similarly repeat development and transfer, whereby plural toner images are superposed to form a color image.
In order to realize the aforementioned function of developing the latent image formed on the photosensitive body, the above-described developing devices include a developing roller as a developer carrier, a toner container, a toner supply roller, and a regulation blade as a developer charging member or the like. Additionally, the developing roller includes the function of carrying the toner and opposes the photosensitive body in order to appropriately develop, with the toner, the latent image formed on the photosensitive body. Also, in a format that conducts development in a state that the developing roller and the photosensitive body are not in contact, such as a jumping development format, a gap is present between the developing roller and the photosensitive body.
Incidentally, for whatever reason, there are cases where the space of the gap at the longitudinal-direction end portions of the developing roller is larger than the space at the longitudinal-direction center portion.
For example, the above-described situation arises when the developing roller is bent due to the regulation blade or the toner supply roller, which abuts against the developing roller along the longitudinal direction of the developing roller, pressing the developing roller towards the photosensitive body.
Also, although the developing roller rotates around the central axis thereof, deflection occurs in the developing roller due to this rotation. Due to this deflection, the space of the gap fluctuates by the rotational period of the developing roller.
The image density of the image formed by development fluctuates in accordance with the fluctuation in the space of the gap. However, whereas the fluctuation in the image density at the longitudinal-direction center portion of the developing roller is minute stemming from the fact that the space of the gap is small, the image density at the longitudinal-direction end portions fluctuates relatively largely stemming from the fact that the space of the gap is large. Additionally, the fluctuation in image density leads to unevenness of density having a periodicity and becomes visible.
Thus, there is a need for a method for realizing a developing roller that is suitable for suppressing the occurrence of uneven density appearing in the image.
Additionally, the regulation blade is fixed, by spot welding, to a support member for supporting the regulation blade and is attached to the developing device via the support member. In this state, the regulation blade abuts against the developing roller, imparts a charge to the toner carried on the developing roller, and regulates the layer thickness of the toner carried on the developing roller.
In a case that the regulation blade is fixed by spot welding to the support member, there are cases where the spot-welded position on the regulation blade determines the free length of the regulation blade. In this instance, the distance between the spot-welded position and the free end of the regulation blade in the lateral direction of the regulation blade becomes the free length. Additionally, fixing the free length along the longitudinal direction of the regulation blade contributes to the evenness of the pressure of the regulation blade on the developing roller.
However, because spot welding is ordinarily done by welding at plural points, of the positions in the longitudinal direction of the regulation blade, the pressure differs between the positions where spot welding has been conducted and the positions where spot welding has not been conducted, and there is the potential for the pressure in the longitudinal direction to become uneven.
Additionally, because the regulation blade is fixed to the support member, the rigidity of the support member is weak, and if flexure occurs in the support member, there is the potential for flexure to also occur in the regulation blade and for the pressure to become uneven.
Further, there is the potential for uneven pressure of the regulation blade arising as a result of these causes being compounded to cause the charge of the toner to become uneven, and such unevenness of the charge triggers drawbacks such as image deterioration, toner leakage and toner scattering.
Thus, in order to make the charge of the toner even, there is a need for a method for reducing unevenness of the pressure of the regulation blade on the developing roller.
Explanation is made on another structure of a conventional laser beam printer provided with a rotary development unit (see Japanese Patent Publications JP 2002-268319A, JP H10-3248A, and JP H08-129306A, for example). Similarly to the foregoing examples, such the laser beam printer has a rotary development unit, and the rotary development unit is loaded with a plurality of developing cartridges served as developing device respectively containing the different colors of toner. During printing, a required color of the rotary developing cartridge is positioned adjacent to the photosensitive body by rotating the rotary development unit, to cause the latent image formed on the photosensitive body to carry toner thereon. The toner is transferred from the photosensitive body through a transfer belt onto a sheet (printing medium) where fixed thereon.
The developing cartridge is provided with a supply roller and a developing roller. The toner is supplied to the developing roller through the supply roller, and then delivered from the developing roller onto the photosensitive body.
The developing cartridge generates heat due to rotational friction of the feed and developing rollers or by driving the other drive systems. If such heat accumulates within the developing cartridge, the toner is denatured by temperature rise, thus causing an adverse effect upon the quality of printing. Particularly, the developing roller, made of metal and hence small in specific heat, is ready to be hot in its surface. Moreover, because the toner delivered from the supply roller exists as a thin layer on the developing roller, the toner is readily denatured by heat.
Meanwhile, a helical gear is provided on the rotation shaft of the developing roller. By the action of the idle gear structured likewise by a helical gear in mesh therewith, the developing roller is urged toward the side of the gear position. By bringing the developing cartridge into abutment against an end abutment part, the developing cartridge is determined for its lengthwise position.
In the meanwhile, the developing cartridge is usually of a plastic material in a thermoplastic nature. Consequently, there encounters softening by the heat of friction or the like caused upon driving various constituent elements including rotating gears, readily causing a deformation in the housing part under the application of a load. Particularly, the support shaft of the idle gear is supported by end faces of the developing cartridge. When the developing roller is urged toward the gear by the action of the helical gear, the reaction of which causes the support shaft of the idle gear to act in a manner pushing the developing cartridge at its end face. With such a load application, there is a problem that the thermally softened end face of the developing cartridge is pushed into deformation.
Meanwhile, with this structure, because the end face of the developing roller is always in pressure-contact with the end abutment part, frictional heat is caused at between the developing roller end face and the end-abutment surface during rotation of the developing roller. The end abutment part, usually structured by a thermoplastic member, possibly softens into deformation due to the frictional heat. This results in a positional deviation of the developing roller in the lengthwise direction. Meanwhile, in case the developing roller end face and the end abutment part have a great frictional resistance, any of these are worn out. This can form a cause of a positional deviation of the developing roller or a cause to impede stable rotation of the developing roller.
Additionally, in order to adsorb toner on a peripheral surface, the supply roller and developing roller is pressure-contacted with an electric contact at an end face of the rotation shaft thereof, to which a charging bias voltage is applied thereby causing charge on the peripheral surface of the roller.
The conventional electric contact is formed nearly in the center of a linear-formed conductor plate. Because the both ends of the conductor plate are fixed to the end faces of the developing cartridge by screws or the like, in case the screw tightening force is strong at the both ends of the conductor plate, the electric contact positioned in the center is strongly pressed directly on the end face of the rotation shaft. Consequently, when the rotation shaft is rotated, there encounters a problem that the electric contact is overheated by the friction between the rotation-shaft end face and the electric contact or the electric contact is worn out into an open hole. In case the screw tightening force is weakened in order to solve the problem, there is a reduction in the contact force of the electric contact with the rotation-shaft end face, possibly causing a problem of shutting down of energization during service.
On the other hands, in such the conventional developing device, at a boundary portion between a developing region of the developing roller and a non-developing region (both ends of the developing roller), a seal member is provided in a gap between the developing roller and the developing device body thereby to prevent the toner around the developing roller in the developing region from leaking to the non-developing region (for example, JP H06-230665A).
However, the above conventional developing device has the following problems: The toner leaks from a small gap between the toner layer regulation blade and the seal member, and the toner disperses by centrifugal force caused by rotation of the developing roller, so that the inside of the device body is stained. Further, the toner is accumulated at both ends of the developing roller, and this accumulated toner is attached onto the photosensitive body and transferred onto both ends of a sheet.
In order to prevent these problems, in the above JP H06-230665A, a magnetism generator is arranged between the seal member and a developer container or the toner layer regulation blade, whereby the toner leakage and the toner dispersion are prevented. However, this causes troublesome assembly work and increase of cost.
Further, in such the developing device, a system is known in which the developing roller is rotatably held with the predetermined gap from the photosensitive body, preferably with a gap of 0.2 mm to 0.5 mm. For example, in JP H12-267439A, to both ends of the developing roller, distance keeping members having the thickness of 0.2 mm to 0.5 mm are rotatably fit, and the developing roller is brought into contact with the photosensitive body.
However, in the above conventional developing device, the frictional power between the developing roller and the distance keeping member increases by long use, and sliding performance of the distance keeping member lowers. Therefore, unevenness in speed is produced in the photosensitive body through the distance keeping member, so that there is a problem that a bad image is produced. Particularly, in case that a peripheral speed of the developing roller is set higher than the peripheral speed of the photosensitive body, this problem becomes large.